There is a rapidly increasing trend towards the use of linerless thermoplastic closures for both glass and plastic bottles. The linerless closure is less expensive than a lined plastic closure and is expected to be less expensive than metal closures which have high energy requirements for their production. The linerless closure is also able to provide a high fidelity seal which makes it an excellent candidate for use in packaging food and beverages. This market is growing at a rapid rate and is expected to have a volume potential so great that it would be presumptious to attempt to project the volume in the next few years.
Due to the short skirts which are characteristic of today's caps, the greatest problem impeding the use of linerless closures is that the short skirts do not provide sufficient surface area for cap thread and bottle thread contact. Insufficiency in thread contact results in the closure backing off and loosening from the bottle as time elapses and as the package is sent through commerce. A system described in U.S. Pat. No. 3,480,170 provides some anti-backoff characteristics by increasing the radial interference between the cap and bottle finish. However, this system is of a fairly complex nature.
It is therefore an object of this invention to provide a thermoplastic closure which exhibits high anti-backoff characteristics and which is simple in construction. It is also another object of this invention to provide a linerless thermoplastic cap having high anti-backoff characteristics.